The effect of location of muscle spindle on the predicted reflex control of muscle length.

The location of muscle spindles is such that the length transduced by their end-organs may not represent the total 'origin-to-insertion' length of the muscle. The tendons at least lie in series with the spindles. Using a muscle model based on biceps brachii in man, a length-control senrosystem utilizing feedback either from the total muscle length, or from the 'internal length' of the contractile central region, is modelled on the analog computer. The difference in the nature of the closed-loop regulatory system thus achieved is demonstrated. Anomalous behavior is seen, in that as the length feedback becomes effective the total system response approaches that of a simple mass/spring system in one instance.